Marketable Securities - Summary of Available-for-sale Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Summary of available-for-sale securities
Available-for-sale securities, Cost	$ 52,671	$ 55,200
Available-for-sale securities, Estimated Fair Value	52,919	55,436
Level 1 | Certificates of deposit
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	1,362	1,690
Unrecognized losses for less than 12 months, Estimated Fair value	1,345	1,677
Unrecognized losses for more than 12 months, Cost	359	110
Unrecognized losses for more than 12 months, Estimated Fair value	355	110
Unrecognized gains, Cost	6,645	6,298
Unrecognized gains, Estimated Fair value	6,681	6,334
Available-for-sale securities, Cost	8,366	8,098
Available-for-sale securities, Estimated Fair Value	8,381	8,121
Level 2 | U.S. Treasury and agency notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	3,289	4,672
Unrecognized losses for less than 12 months, Estimated Fair value	3,281	4,664
Unrecognized losses for more than 12 months, Cost	1,702	1,699
Unrecognized losses for more than 12 months, Estimated Fair value	1,699	1,694
Unrecognized gains, Cost	4,581	3,713
Unrecognized gains, Estimated Fair value	4,621	3,760
Available-for-sale securities, Cost	9,572	10,084
Available-for-sale securities, Estimated Fair Value	9,601	10,118
Level 2 | Corporate notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	4,105	7,270
Unrecognized losses for less than 12 months, Estimated Fair value	4,076	7,192
Unrecognized losses for more than 12 months, Cost	1,796	530
Unrecognized losses for more than 12 months, Estimated Fair value	1,787	523
Unrecognized gains, Cost	21,926	21,945
Unrecognized gains, Estimated Fair value	22,123	22,173
Available-for-sale securities, Cost	27,827	29,745
Available-for-sale securities, Estimated Fair Value	27,986	29,888
Level 2 | Municipal notes
Summary of available-for-sale securities
Unrecognized losses for less than 12 months, Cost	524	1,688
Unrecognized losses for less than 12 months, Estimated Fair value	520	1,677
Unrecognized losses for more than 12 months, Cost	249	318
Unrecognized losses for more than 12 months, Estimated Fair value	247	316
Unrecognized gains, Cost	6,133	5,267
Unrecognized gains, Estimated Fair value	6,184	5,316
Available-for-sale securities, Cost	6,906	7,273
Available-for-sale securities, Estimated Fair Value	$ 6,951	$ 7,309